This is a continuation of application Ser. No. 42,623 filed on Jan. 11, 1988, now abandoned.
The invention relates to improvements in methods and apparatus for preparing wood in the process of preparation of pulp for papermaking and particularly in the preparation of wood chips.
In the paper industry, wood pulp is made by subjecting wood chips to a chemical digestion process wherein the compounds and chemical systems holding the fibers together, such as lignin, are dissolved to thereby liberate the individual wood fibers which are then diluted with water and introduced into a papermaking machine to make paper or paperboard products. The preparation of the wood chips is a critical part of the process in that, ideally, the wood chips should be of uniform thickness so that the chemicals penetrate the chips equally during the digestion process. If not properly formed, the chips may not be sufficiently penetrated to liberate the wood fibers, or if the chips are too thin, the chemicals may penetrate the chips too rapidly or for a longer time than necessary to liberate the individual fibers. The fibers themselves may be deleteriously weakened, or shortened or both.
The thickness of the individual wood chips is defined in the direction extending radially relative to the longitudinal axis of the log. Control of cutting the chips is difficult since the chips are sometimes gouged or broken out in chunks as the result of knots and compression. To ensure consistent chip size, screening systems are used to separate out oversize chips, which are directed to apparatus for reducing the size of the chips. Commercial type wood rechippers take various forms, and one successful form has been a rotary rechipper or chip slicer having a drum in which a rotor with vanes carries the wood in a rotary path to force the wood against knives circumferentially placed around the rotor. The rotor carries anvil vanes and rotates at a controllable speed on the order of 200 to 1000 rpm. As the wood is fed near the center of the anvil and enters the spaces between the rotor vanes, the wood is impacted by the vanes, preventing it from entering further into the length of the drum and is forced radially against the knives. Unless the wood is centered axially along the rotary anvil, uneven loading along the length of the drum occurs so that the drum is heavily loaded in the front or entry side and lightly loaded at the rear side, when viewing the rotary anvil with respect to its axial load. The nonuniform axial loading of the anvil contributes to nonuniformity in chip quality or size and tends to reduce the quality of the pulp produced. A need exists to produce chips of maximum uniformity and to minimize fines generated by chip slicing equipment. Wider slicers have been utilized to achieve more uniform chips, but higher wear and lower capacities than expected have resulted. This is due to a large extent upon nonuniform loading in an axial direction and this problem also exists on narrow units as well, but to a lesser degree.
It is accordingly an object of the present invention to provide an apparatus and method utilizing a rotary chip slicer which is capable of producing more uniform chips and is capable of long periods of operation without repair or adjustment.
A further object of the invention is to provide a chip slicer which has higher wear capability and higher capacity than heretofore available.
A still further object of the invention is to provide a rotary chip slicer for the production of wood chips in a pulping process wherein an axial anvil type rotor is used for slicing the chips and a uniform axial load is attained along the length of the rotary anvil making it possible to use a wider or longer anvil than heretofore available.